01 - Whisk eggs, milk, salt, and pepper together in a mixing bowl until thoroughly blended.
02 - Heat a 10-inch non-stick skillet over medium heat and swirl in unsalted butter to coat the pan evenly.
03 - Add minced garlic and sauté for 30 seconds until aromatic, taking care not to allow browning.
04 - Pour the egg mixture into the skillet and cook gently, stirring with a spatula, until eggs are beginning to set but remain slightly runny on top, about 2 minutes.
05 - Distribute cheddar and mozzarella evenly over the eggs and sprinkle with parsley if desired.
06 - Place the flour tortilla directly on top of the eggs, pressing gently, and continue cooking for 1 to 2 minutes until the eggs are mostly set.
07 - Using a large spatula or by inverting onto a plate, carefully flip the omelette so the tortilla is in contact with the pan.
08 - Cook for an additional 1 to 2 minutes until the tortilla is crisp and golden brown.
09 - Transfer to a plate, fold in half, slice, and serve immediately while hot.